UV lamps are often used to purify water. The lamp is usually mounted in a chamber and water flows from an inlet into the chamber, around the lamp, and then out of the chamber through an exit after being sterilized by incident UV radiation emitted by the lamp.
The bacteriocidal effect occurs at certain wavelengths of UV radiation and radiation levels above prescribed power levels. The wavelength requirements are defined by the UV lamp employed, but due to aging the power level changes over a period of time. To ensure safe operation a monitor is required to indicate the power level or intensity of radiation from the lamp.
To achieve an effective bacteriocidal action a very high intensity lamp is used. The transmission of glass is seriously affected over a period of time and the power levels are generally too high for linear operation of normal photo detectors. The system must reduce the intensity of the radiation to practical levels.
The lamps used are generally of the medium pressure kind which although they emit high level radiation, that radiation is not confined to discrete wave-lengths. The radiation usually extends into the visible region. As the aging process of the lamp progresses the UV portion of the spectrum declines more rapidly than the visible portion. The detection system used must therefore respond primarily to the UV portion below 300 nm. Optical filters for this region are very expensive and thus unattractive to use commercially.